What does "duo" format mean in music?

I’m talking in terms of the usual music industry/scene.

Does “duo” usually mean only two members (musicians) composing, arranging, and playing ALL the songs (say for example: “acoustic guitar duo”), or it could actually mean two permanent members HELPED by additional/session musicians?

Then suppose if I compose, arrange all the songs, and also act as a singer/keyboardist-pianist, and have already found a perfect, matching great female vocalist.
and NOT all the songs would have guitar, bass, and drum (ie: band format), some songs would be in acoustic, some songs would be in orchestra/symphonic form, and some in electronica forms,
then is my best bet would be just to form a “duo”, as opposed to have to find another 3 to 4 band members ie: lead guitar, rhythm guitar, bass guitar, drums, thus more hassle?

Last but not least, how does the royalty distribution goes with a “duo” format?
this including also the additional/session band members. how does the ‘payment system’ usually work?
thx.